In wireless communication system, a signal transmitted from an antenna typically follows multiple paths to its destination. In many communication systems, these paths are combined at the receiver to produce a received signal with a better signal quality than any one of the paths alone could provide. One approach to combine these multiple paths is a Rake receiver which combines a specified number of the stronger paths together. The Rake receiver recovers the received signal over each of the strong paths, weights each recovered path signal by a magnitude and phase and combines the resulting weighted signals together.
To determine the paths to combine and the corresponding weights to use for those paths, a path searcher is typically used. The path searcher typically searches code phases for multipath components of a transmitted signal. The code phases having the strongest received components are selected for the Rake receiver. Based on the received energy of these components, the path searcher determines the magnitude each component should be given by the Rake.
FIGS. 1A and 1B are simplified illustrations of cells 24A and 24B that a path searcher may be utilized. These figures have been extremely simplified for illustrative purposes. In FIG. 1A, cell 24A is unsectorized. The base station 20 uses one antenna element to receive signals from each user, UEs 221 to 223. A path searcher in cell 24A analyzes each user over its delay spread plus some margin for uncertainty, such as 100 chips. As a result, only a spread of 300 chips is analyzed by the path searcher of cell 24A.
By contrast, in FIG. 1B, the cell 24B has six sectors 271 to 276. The base station 20 of the cell 24A uses two antenna elements 2611 to 2662 per sector 271 to 276. For UE 222, the paths of its transmissions are analyzed by each antenna element 2661, 2662 of its sector 276 over the delay spread of that UE's transmissions, such as 100 chips. As a result, effectively a combined spread of 200 chips (100 chips per antenna) is analyzed for UE 222. UE 221 is moving between sectors 271 and 272 and is experiencing softer handover. In softer handover, the base station 20 receives the UE's transmissions over both sector's antenna elements 2611, 2612, 2661, 2662. For a UE experiencing softer handover in cell 24B, effectively a combined spread of 400 chips (100 chips per two antenna elements per two sectors) is analyzed. To analyze the paths of all the UEs 221 to 227 of cell 24B, a combined delay spread of 1600 chips (6 users in one sector, 200 chips, and one user experiencing softer handover, 400 chips) is analyzed by the path searcher.
As illustrated by FIGS. 1A and 1B, a path searcher for the Node-B/base station of cell 24A needs to analyze far less paths than a path searcher for cell 24B. One approach to design a path searcher to handle both cells is to design the path searcher for the worst case loading, such as cell 24B. One drawback to such a path searcher is that much of its resources are not utilized when used in a lightly loaded cell, such as cell 24A. As a result, the operator of cell 24A may invest in an inefficient path searcher more costly than necessary.
Another approach is to design a path searcher customized to each cell. One path searcher design handles lightly loaded cells, such as cell 24A. Another path searcher design handles heavily loaded cells, such as cell 24B. Although such an approach minimizes the amount of idle resources in lightly loaded cells, it requires two or multiple differing designs, which is undesirable. Additionally, cell loadings may change over time. The loading of cell 24A may increase in loading to the level of cell 24B. In such a situation, the lightly loaded path searcher would be replaced by a heavily load cell path searcher. Such a retrofit is costly and undesirable.
Accordingly, it is desirable to have a Node-B/base station path searcher adaptable to varying cell conditions.